Adaptive optical system technology has found a wide range of applications including astronomical imaging and long-range free space optical communication. Adaptive optical system technology can potentially enhance any application in which turbulence occurs along the path, leading to refractive index fluctuations due to temperature variations. This turbulence degrades the performance of an imaging or laser projection system. Prior art methods (Ref. 1-15), as referenced in the references cited section below, have been developed to provide methods for compensation of the effects of turbulence on laser propagation through the atmosphere. In particular, the method described jointly by Refs. 14 and 15, was developed to jointly provide compensation of aberrations in both the laser source and those aberrations induced by propagation through a turbulent medium. The method described in Ref. 15 requires two phase correction devices to effect this joint compensation. The method described in Refs. 14 and 15 offers the particular advantage of nulling aberrations in the “Aperture Sharing Element” or ASE. The ASE is an optical element in a laser beam projection system that is typically highly reflective at the wavelength and/or polarization of the outgoing laser beam but is highly transmissive at the wavelength and/or polarization of the laser beam received from the direction of the target (either cooperative or non-cooperative) that is used for wavefront sensing to measure the aberrations in the turbulent medium. Aberrations in the ASE can be significant, particularly in the case of applications in which the projected laser beam is a High Energy Laser (HEL) and can induce thermal aberrations in the ASE. A disadvantage of the method in Ref. 14 is that the aberrations in the HEL and ASE are measured in an “open loop” fashion without feedback. This leads to significant errors when the system is not perfectly aligned and/or can lead to non-trivial calibration errors. A disadvantage of the method in Ref. 15 is that two phase correction devices are required in the beam path of the projected laser beam. Particularly for applications that project a HEL the phase correction device becomes a critical single point failure for the laser beam projection system. Furthermore, phase correction devices suitable for use in a high power beam path are very expensive. In addition, the drive electronics for a phase correction device that is suitable for use in a high power beam path are typically heavy, bulky, and consume a significant amount of power.
What is needed is a method for joint compensation of aberrations induced by both propagation through a turbulent medium and the aberrations in a laser beam (including the aberrations induced by reflection from the ASE) utilizing only a single phase correction device in the projected laser beam path while at the same time the corrections are made using strictly null-seeking feedback control loops, thus providing a robust means of compensation. The present invention meets these needs by providing a configuration that provides joint compensation of aberrations induced by propagation through a turbulent medium and the aberrations in a laser beam (including the aberrations induced by reflection from the ASE) utilizing only a single phase correction device in the projected laser beam path and a secondary phase correction device incorporated into the wavefront sensor path while at the same time the corrections are made using strictly null-seeking feedback control loops, thus providing a robust means of compensation. The second phase correction device operates in the sensor path with very low incident optical power and thus can be a very compact device with minimal cost, minimal weight and minimal power consumption requirements. The present invention utilizes two wavefront sensors: one to measure the aberrations in the path from the ASE to the target and a second to measure the aberrations in the laser beam source and the aberrations induced by propagation from the laser beam source to the ASE (including reflection from the ASE). Any method for wavefront sensing that is appropriate to measure the induced aberrations is compatible for use with the present invention.
U.S. Pat. No. 8,076,624 issued Dec. 13, 2011 from U.S. patent application Ser. No. 12/324,041 filed Sep. 19, 2008 is fully incorporated herein by reference and provides a wavefront sensing and control technique to measure the aberrations along the propagation path from the target to the ASE using return from an ultra-short coherence length laser forming a controllable focused laser beacon at a non-cooperative target, regardless of the surface depth of the target.
U.S. patent application Ser. No. 12/962,163 filed Dec. 7, 2010 is fully incorporated herein by reference and provides a wavefront sensing and control technique to measure the aberrations along the propagation path from the target to the ASE using return from the combination of a short pulse laser source and the depth of the target surface at the aimpoint to form a controllable focused laser beacon at the target.
U.S. patent application Ser. No. 13/732,793 filed Jan. 2, 2013 is fully incorporated herein by reference and provides a wavefront sensing and control technique to measure the aberrations along the propagation path from the target to the ASE using return from the combination of a short coherence length laser source and the depth of the target surface at the aimpoint to form a controllable focused laser beacon at the target.
U.S. patent application Ser. No. 14/283,463 filed May 21, 2014 is fully incorporated herein by reference and provides a wavefront sensing and control technique to measure the aberrations along the propagation path from the target to the ASE that minimizes signal required for the wavefront sensor.
U.S. Pat. No. 4,635,299 issued Jan. 6, 1987 is fully incorporated herein by reference and provides a system and method for providing measurement and compensation of aberrations in a laser beam projected from a laser source and the aberrations induced by propagation through an optical system and through a turbulent medium to a target.
The present invention provides for an alternate system and method for providing measurement and compensation of aberrations in a laser beam projected from a laser source and the aberrations induced by propagation through an optical system and through a turbulent medium to a target utilizing only a single primary phase correction device in the projected laser beam path and a secondary phase correction device incorporated into the wavefront sensor path.